Apparatus for blow molding

ABSTRACT

Expanding the parison while the mold side parts defining the walls of the blow-molding chamber, one part per wall, are moved to mold closed positions causes flash to be pinched between each mold part and provides good wall thickness near the edges of the final product.

BACKGROUND OF THE INVENTION

This application is a division of copending application Ser. No.728,912, filed Apr. 30, 1985, now U.S. Pat. No. 4,650,627 issued Mar.17, 1987.

In one aspect, the invention relates to an apparatus suitable forforming a blow molded article. In another aspect, the invention relatesto a method for blow molding a plastic article.

Blow molding processes are widely used to form hollow articles fromplastic materials. Generally, a parison, which is a tube shaped blank ofmolten plastic material, is extruded from a nozzle having a generallyannular opening defined between a die ring and land and is directed bygravity to between the parts of an opened mold. When a desired length ofparison has been extruded, the mold is closed and compressed gas,usually air, is injected into the parison to pneumatically expand it tofit the inside contours of the mold, the article thus formed is cooledto stabilize its form, and the mold is opened to retrieve the article.When hollow articles having a cross section different substantially fromthe circular are to be blow molded, the sides of thecircular-cross-sectioned parison are expanded a different degree sothere occurs a thinning of the plastic material as the parison isstretched. Because the parison must be stretched the greatest extent inorder to reach crevices, corners and edges, it is in these areas oflocalized concavities where weaknesses and flaws in blow molded articlesare most likely to occur. The problem is especially severe where theblow molded article is to have sharp corners.

Two procedures have been used in the prior art to reduce the higharticle scrap rate which associates itself with blow molding articleshaving sharp corners. The first is to blow mold articles having roundedcorners. This reduces but does not eliminate the problem since localizedparison thinning occurs even in rounded corners. Also, articles havingrounded corners are not necessarily as commercially desirable asarticles having sharp corners since they do not pack as well. The othertechnique involves providing a greater amount of material to the cornersand edges of the article by localized adjustment to the gap between thedie ring and land. For example, providing the gap between the ring andland with a notch allows for a thickened rib of material on the parisonwhich can be oriented with respect to the mold to provide a largeramount of material to longitudinally extending edges. By programming thedistance between the die ring and land with time as the parison isdropped, thickened rings on the parison can be provided to correspond tocircumferentially extending edges in the mold. While these approachesprovide some benefits, processes for controlling wall thicknesseswithout using larger amounts of material would clearly be verydesirable.

OBJECTS OF THE INVENTION

It is an object of this invention to provide an apparatus for forming ablow molded article having angular edges.

It is a further object of this invention to provide a method for blowmolding a plastic article having angular edges.

SUMMARY OF THE INVENTION

In one aspect of the invention, an apparatus is provided for forming ablow molded article. The apparatus comprises a means for extruding amolten tubular parison from between a die and a ring into a closeablemold. The tubular parison is attached to the means for extruding by itsfirst end and the second end is pinched shut by a suitable means priorto the closing of the mold. The parison is pneumatically expanded bysuitable means as the mold is closed. The mold, when in a closedposition, defines a boxed shaped chamber having angular corners andedges determined by a top surface, a bottom surface, and a plurality ofside surfaces. In accordance with the first embodiment of the invention,at least one of the top surface and the bottom surface of the mold isdetermined by a pair of mold parts.

In another aspect of the invention, there is provided a method for blowmolding a plastic article from a parison. The method comprises extrudinga molten tubular parison between a plurality of mold parts. The open endof the parison is pinched shut. The parison is then pneumaticallyexpanded. The parison is then pinched between a first pair of mold partsat a first longitudinal position and pinched between a second pair ofmold parts at a second longitudinal position spaced apart from the firstlongitudinal position. At least three mold side parts are then movedradially inward toward the longitudinal axis of the parison toward amold closed position while pneumatic expansion of the parison iscontinued. In this manner, flash lines are formed between the first pairof mold parts, the second pair of mold parts, each of the side moldparts, and between the first and second pairs of mold parts and the moldside parts. By bringing the sides of the mold in so that they pinchflash where the parison has a better thickness, a part with good walldistribution and strength in the corners is produced. The flash ofcourse can be trimmed off and recycled in a conventional manner.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 schematically illustrates layout of the inside surfaces of a blowmold in the closed position of the outside surfaces of an article formedby the mold.

FIG. 2 is a side view illustrating certain features of an embodiment ofthe invention. The mold surfaces which define certain of the surfacesshown in FIG. 1 when in a mold closed position are called out by 100greater than the surfaces identified in FIG. 1.

FIG. 3 schematically illustrates the device of FIG. 2 at a point laterin time with certain mold parts in the mold closed position.

FIG. 4 illustrates schematically certain other aspects of the inventionwhen seen along lines 4--4 of FIG. 3. The mold surfaces used todetermine the article surfaces in FIG. 1 are identified by call outs 100greater than the corresponding surfaces in FIG. 1.

FIG. 5 is a side sectional view of a mold as shown in FIG. 4 when viewedalong lines 5--5 in the mold closed position. The mold surfacesdetermining the article surfaces are identified by call outs 100 greaterthan the corresponding call outs in FIG. 1.

FIG. 6 illustrates a different mold from FIGS. 2 through 5 in anotherembodiment of the invention viewed from the perspective of FIG. 4 but ina mold closed position with the article formed within the mold. Articleshaving certain unique corner geometries with good wall distribution canbe formed in accordance with the invention.

FIG. 7 is an illustration of a plastic article removed from the moldchamber of FIGS. 1 through 5 with the flashing thereon.

DETAILED DESCRIPTION OF THE INVENTION

Conventional blow molding of deep draw parts with sharp corners rangesfrom difficult to impossible with state of the art technology. For thisreason, blow molded parts normally have generous radii in the edges andcorners. If a part has much more than a 1:1 draw ratio (more than 1× asdeep as it is wide), it normally cannot be blow molded. At this point,the given design must be made by other methods.

The invention relates to a method and apparatus for blow molding deepdraw parts with sharp corners which cannot be produced with today'stechniques. The process and apparatus employ a moving sectioned moldcombined with a prepinched and preblown parison. Conventional technologycan be used to actuate the moving sections of the mold, drop theparison, prepinch it, and preblow it. For example, the mold sections canbe actuated pneumatically, hydraulically, or by cam action. The moldside parts have not been shown in FIGS. 2 and 3.

Referring to FIG. 2, an apparatus 112 is schematically illustrated. Theapparatus comprises a means 114 for extruding a molten tubular parison116 from between a die and a ring into a closeable mold 118. The tubularparison 116 has a first end 120 attached to the means 114 so that theparison 116 depends from it and a second end 122. A means 124 isprovided for pinching closed the second end of the tubular parison priorto closing the mold 118. In the illustrated embodiment, the means 124 isprovided between the mold 118 and the extruder means 114 although themold 118 can be positioned between the means 124 and the extruder 114 ifdesired. A means 126 is provided for pneumatically preexpanding theparison prior to closing the mold 118. Generally, the means 126comprises a tube having a nozzle at the end and connected to a source ofcompressed gas such as air. The tube is movable axially along the axisof the tubular parison and is extended so that the nozzle end is insideof the mold when the mold is in a closed position in order to providegas to urge the parison against the side walls of the mold by pneumaticexpansion. The mold 118, when in a closed position, defines a boxedshaped chamber which can be as illustrated in FIG. 1. The chamber hasangular corners such as 128 and 130 and edges such as 132 and 134 whichwould be extremely difficult to form with good wall distribution withoutusing the present invention. The chamber is determined by a top surfaceformed by sections 1a and 1b, a bottom surface formed by sections 6a and6b and a plurality of side surfaces 2, 3, 4 and 5.

In accordance with the first embodiment of the invention, at least oneof the top surfaces 1a and 1b and the bottom surfaces 6a and 6b isdetermined by a pair 201 of mold parts a pair 206 of mold parts and/or106a and 106b as best illustrated in FIGS. 2 and 3.

In a preferred embodiment of the invention, the top surface 101 isdetermined by a first pair 201 of mold parts and the bottom surface 106is determined by a second pair 206 of mold parts. The first pair 201 ofmold parts comprises a first mold part 201a and a second mold part 201b.The first mold part 201a of the first pair has a first face 101a and asecond face 136. The second mold part 201b of the first pair 201 has afirst face 101b and a second face 138 which matches the second face 136of the first mold part 201a. The first face 101a of the first mold part201a of the first pair 201 and the first face 101b of the second moldpart 201b of the first pair 201 together determine the top surface 101of the mold chamber when the mold 118 is in the closed position.

The second pair 206 of mold parts comprises a first mold part 206a and asecond mold part 206b. The first mold part 206a of the second pair 206has a first face 106a and a second face 140. The second mold part 206bof the second pair 206 has a first face 106b and a second face 142 whichmatches the second face 140 of the first mold part 206a of the secondpair 206. The first face 106a of the first mold part 206a of the secondpair 206 and the first face 106b of the second mold part 206b of thesecond pair 206 together determine the bottom surface 106 of the mold118 when the mold is in the closed position.

A means 144 is provided for moving the first mold part 201a of the firstpair 201 from a mold open position (FIG. 2) to a mold closed position(FIG. 3). A means 146 is provided for moving the second mold part 201bof the first pair 201 from a mold open position (FIG. 2) to a moldclosed position (FIG. 3). When in the mold closed position, the secondface 136 of the first mold part 201a of the first pair 201 is closelyadjacent to the second face 138 of the second mold part 201b of thefirst pair 201. The means 126 generally extends through a boreholedefined between the mold parts 201a and 201b. A means 148 is providedfor moving the first mold part 206a of the second pair 206 from a moldopen position (FIG. 2) to a mold closed position (FIG. 3). A means 150is provided for moving the second mold part 206b of the second pair of206 from a mold open position (FIG. 2) to a mold closed position (FIG.3). When in the mold closed positions, the second face 140 of the firstmold part 206a of the second pair 206 is closely adjacent to the secondface 142 of the second mold part 206b of the second pair 206.

The sides, 2, 3, 4 and 5 of the FIG. 1 article are determined, withreference to FIG. 4, by surfaces 102, 103, 104 and 105 of mold sideparts 202, 203, 204, and 205 respectively. A first mold side part 202connects the top surface 101 of the mold with the bottom surface 106when positioned in a mold closed position. A means 152 is provided formoving the first mold side part 202 from a mold open to a mold closedposition. A second mold side part 205 is opposed from the first moldside part 202. The second mold side part connects the top surface 101 ofthe mold with the bottom surface 106 when positioned in a mold closedposition. A means 154 is provided for moving said second mold side part205 from a mold open to a mold closed position. A third mold side part203 connects the first mold side part 202 and the second mold side part205 and the top surface 101 of the mold with the bottom surface 106 whenpositioned in a mold closed position. A means 156 is provided for movingsaid third mold side part 203 from a mold open to a mold closedposition. A fourth mold side part 204 is opposed from the third moldside part 203 and connects the top surface 101 of the mold with thebottom surface 106 when positioned in the mold closed position as wellas the mold side part 202 with the mold side part 205. A means 158 isprovided for moving the fourth mold side part from a mold open positionto a mold closed position.

With reference to FIG. 5, in a preferred embodiment, each mold part201a, 201b of the first pair 201 of mold parts is provided with a thirdface 160, 162 opposite to the second face 136, 138. Each mold part 206a,206b of the second pair 206 of mold parts is provided with a third face164, 166 opposite to the second face 140, 142. The fourth mold side part204 has a first face 104 which determines the side of the mold chamber,a second face 168 which matches the third face 160 of the first moldpart 201a of the first pair 201 and a third face 170 which matches thethird face 164 of the first mold part 206a of the second pair 206. Thethird mold side part 203 has a first face 103 which determines a sidewall of the mold when the mold part is in the closed position, a secondface 172 which matches the third face 162 of the second mold part 201bof the first pair 201 when the mold parts are in the mold closedposition and a third face 174 which matches the third face 166 of thesecond mold part 206b of the second pair 206 when the mold parts are inthe mold closed position.

In the illustrated embodiment, with reference to FIG. 4 it will beunderstood that the first mold side part 202 contacts a first endsurface 176 of the first mold part 206a of the second pair 206 and afirst end surface 178 of the second mold part 206b of the second pairwhen the mold is in the closed position. In a similar manner (not shownbut would appear the same as FIG. 4) the first mold side part contacts afirst end surface of the first mold part 201a of the first pair 201 anda first end surface of the second mold part 201b of the first pair 201.The first mold side part 202 further is connected to a fourth surface180 of the third mold side part 203 and a fourth surface 182 of thefourth mold side part 204 when the mold is in the closed position. Thesecond mold side part 205 contacts a second end surface 184 of the firstmold part 206a of the second pair 206 and a second end surface 186 ofthe second mold part 206b of the second pair 206. Similarly, (not shown)the mold part 205 contacts a second end surface of the first mold part201a of the first pair of mold parts and a second end of the second moldpart 201b of the first pair of mold parts. The second mold side part 205further contacts a fifth surface 188 of the third mold side part 203 anda fifth surface 190 of the fourth mold side part 204 when the mold is ina closed position.

In FIG. 6, the point 192 illustrates the longitudinal axis of the moldchamber. The mold chamber is determined by a first mold side part 302, asecond mold side part 305 positioned across the chamber from the firstmold side part 302; a third mold side part 303 connecting the mold sidepart 302 with the mold side part 305 when positioned in a mold closedposition; a fourth mold side part 304 positioned across the chamber fromthe third mold side part 303 and connecting the mold part 302 with themold part 305 when the mold parts are positioned in the closed position.The mold will also include an upper pair of mold parts and a lower pairof mold parts, which can be as illustrated in FIGS. 2 and 3. Thelongitudinally extending edges 310, 312, 314 and 316 of the article 318are provided with various configurations to illustrate the scope of thepresent invention. Good wall distribution is obtained by pinching theparison to form flash 320, 322, 324 and 326 at each of the edges 310,312, 314 and 316, respectively. It is believed that the usual practiceof the invention will best be carried out by moving the mold parts 302,303, 305 and 304 radially inwardly with respect to the axis 192 as theparison is expanded radially outwardly to obtain good wall distribution.

Using the above-described apparatus provides a method for blow molding aplastic article from a parison. The molten tubular parison is extrudedor dropped between a plurality of mold parts. The open end is pinchedshut. The parison is pneumatically preexpanded such as via means 126,pinched between a first pair of mold parts such as the mold parts 101aand 101b at a first longitudinal position and pinched between a secondpair of mold parts such as the mold parts 106a and 106b at a secondlongitudinal position. At least three mold side parts are then movedgenerally radially inwardly toward the longitudinal axis of the parisonwhile pneumatic expansion of the parison is continued and the mold partsultimately reach a mold closed position. According to the invention,flash lines of parison will be formed between the first pair of moldparts, the second pair of mold parts, each of the mold side parts andbetween the first and second pairs of mold parts and the mold sideparts.

In another aspect, the method of the invention comprises blow molding aplastic article having angular edges from a tubular parison whileproviding good wall distribution. The parison is extruded or dropped,and is then preexpanded. At least seven mold parts which determine theshape of the plastic article are moved from mold open to mold closedpositions and a flash line of the parison is pinched between each moldpart. In this manner, a flash line is formed at each angular edge of theplastic article. Where the mold has three side parts the plastic articlewill have at least nine angular edges and flash along each edge. Wherethe mold has four side parts the plastic article will have at leasttwelve angular edges and flash 80 along each edge and top and bottomflash 81 as shown in FIG. 7. The flash 80 and 81 is trimmed off toprovide the finished article 82 with uniform wall distribution and highstrength. For a box-shaped article, at least eight mold parts will berequired to practice the process. The method of the invention is evenmore clearly distinguished from the art where the article to be formedconsists essentially of substantially flat sides. Usually, the parisonwill be formed from polyethylene or polyethylene copolymer sincepolyethylene is cheap and blow molding technology using it is widelypracticed.

What is claimed is:
 1. Apparatus comprising:a closable mold comprisingat least seven mold parts, each mold part being moveable form a moldopen position to a mold closed position, said closable mold, when in themold closed position, defining a box-shaped chamber having angularcorners and edges and at least five sides, wherein the mold parts cometogether when the mold is in the closed position to determine theangular corners and edges of the box-shaped chamber; an extrusion meansfor extruding a molten tubular parison in between the mold parts whenthe mold parts are in the mold open position; an expansion means forpneumatically expanding a molten tubular parison to form a partlyexpanded parison prior to the mold parts being moved from the mold openposition to the mold closed position; and a means for moving the moldparts from the mold open position to the mold closed position so aspinch a portion of a partly expanded parison between adjacent mold partsbeing moved into the mold closed position.
 2. Apparatus as in claim 1wherein the box-shaped chamber has a top surface and a bottomsurface,wherein the top surface is determined by a first pair of moldparts; wherein the bottom surface is determined by a second pair of moldparts; wherein the first pair of mold parts comprises a first mold partand a second mold part; the first mold part of the first pair having afirst face and a second face; the second mold part of the first pairhaving a first face and a second face, said second face of the secondmold part of the first pair of mold parts matching the second face ofthe first mold part of the first pair of mold parts; the first face ofthe first mold part of the first pair and the first face of the secondmold part of the first pair together determining the top surface of themold chamber when the mold is in the closed position; wherein the secondpair of mold parts comprises a first mold part and a second mold part;the first mold part of the second pair having a first face and a secondface; the second mold part of the second pair having a first face and asecond face, said second face of the second mold part of the second pairof mold parts matching the second face of the first mold part of thesecond pair of mold parts, the first face of the first mold part of thesecond pair of mold parts and the first face of the second mold part ofthe second pair of mold parts together determining the bottom surface ofthe mold when the mold is in the closed position.
 3. Apparatus as inclaim 2 further comprisinga means for moving the first mold part of thefirst pair of mold parts from a mold open position to a mold closedposition; a means for moving the second mold part of the first pair ofmold parts from a mold open position to a mold closed position; whereinthe second face of the first mold part of the first pair of mold partsand the second face of the second mold part of the first pair of moldparts are closely adjacent to each other in the mold closed position; ameans for moving the first mold part of the second pair of mold partsfrom a mold open position to a mold closed position; a means for movingthe second mold part of the second pair of mold parts from a mold openposition to a mold closed position; wherein the second face of the firstmold part of the second pair of mold parts and the second face of thesecond mold part of the second pair of mold parts are closely adjacentto each other in the mold closed position.
 4. Apparatus as in claim 2further comprising a first mold side part connecting the top surface ofthe mold with the bottom surface when positioned in a mold closedposition;a means for moving said first mold side part from a mold openposition to a mold closed position; a second mold side part opposed fromsaid first mold side part connecting the top surface of the mold withthe bottom surface when positioned in a mold closed position; a meansfor moving said second mold side part from a mold open position to amold closed position; a third mold side part connecting the first moldside part and the second mold side part and the top surface of the moldwith the bottom surface when positioned in a mold closed position; ameans for moving said third mold side part from a mold open position toa mold closed position; a fourth mold side part opposed from the thirdmold side part connecting the first mold side part and the second moldside part and the top surface of the mold with the bottom surface whenpositioned in a mold closed position; and a means for moving the fourthmold side part from a mold open position to a mold closed position;wherein each of the side parts determines a substantially flat sidesurface of the mold chamber and wherein the chamber has six sidesurfaces.
 5. An apparatus as in claim 4 wherein each mold part of thefirst pair of mold parts and each mold part of the second pair of moldparts is provided with a third face opposite each second face;whereinthe fourth mold side part has a first face which determines a sidewallof the mold when the mold parts are in the mold closed position andsecond face which matches the third face of the first mold part of thefirst pair of mold parts and a third face which matches the third faceof the first mold part of the second pair of mold parts; and wherein thethird mold side part has a first face which determines a sidewall of themold when the mold parts are in the mold closed position, a second facewhich matches the third face of the second mold part of the first pairof mold parts when the mold parts are in the mold closed position, and athird face which matches the third face of the second mold part of thesecond pair of mold parts when the mold parts are in the closedposition.
 6. Apparatus as in claim 5 wherein the first mold side partcontacts a first end surface of the first and second mold parts of thesecond pair, a first end surface of the first and second mold parts ofthe first pair, a fourth surface of the third mold side part, and afourth surface of the fourth mold side part when the mold parts are inthe closed position; and wherein the second mold side part contacts asecond end surface of the first pair of mold parts and a second endsurface of the second pair of mold parts, a fifth surface of the thirdmold side part and a fifth surface of the fourth mold side part when themold is in the closed position.